1. Field of the Invention
The invention relates to a method for manufacturing the impeller that is capable of being mounted on a shaft of an electric motor of a radial compressor or radial blower, and the use of the same.
2. Description of the Background Art
In the prior art, a great number of different types of compressors and blowers are known being used in various applications. Particularly in industrial applications, pumping of gases is accomplished with the help of centrifugal compressors, also known as radial compressors, and radial blowers. Typically this kind of blower construction comprises a rotary impeller, such as shown in FIG. 1, serving to accelerate the gas flow by centrifugal force, and a spiral-shaped flow-guiding envelope construction called the casing. Both the impeller and the casing are typically made from sheet steel or other sheet metal. Also conventionally, the sheet metal construction is comprised of two-dimensional (2D) elements.
In order to reach higher pressures or elevated pumping efficiency, the flow dynamics of the impeller and casing must be improved. To this end, both the impeller and the casing must have a three-dimensional (3D) geometry. As the fabrication of such a construction from sheet elements is impractical, casting is typically employed as the manufacturing method. Besides the choice of the different manufacturing method, a blower construction is complemented with a duct known as the diffuser that is adapted between the impeller and the spiral casing to convert dynamic pressure into static pressure. This kind of construction is generally called a compressor. Annexed FIG. 2 shows a sectional view of a compressor mounted on the shaft of an electric motor.
The above-described approach to reach higher outlet pressure generally requires that the impeller is driven faster than the maximum rotational speed provided by an electric motor. Conventionally, the elevation of impeller rotational speed has been implemented with the help of a step-up gear or V-belt drive. The impeller drive shaft is designed such that the first critical speed of the shaft is situated below the actual rotational speed. This arrangement allows the use of a relatively thin shaft and a heavy impeller. However, the rotational speed of such a compressor is not adjustable inasmuch as the control range would extend to the critical speed that could cause damage to the machinery.
A recent technology in stepping up rotational speed is the use of a frequency converter. Thereby the blower or compressor can be variable-speed controlled. Such a solution allows the impeller to be mounted directly on the shaft of the electric motor without the need for a step-up gear. However, direct mounting on the motor shaft poses certain technical constraints. The construction of the impeller and its drive shaft must be designed such that the first critical speed is situated higher than the maximum drive speed, whereby a relatively thick shaft is required. Yet, there is a limit to the use of a thick shaft, because the diameter of bearings also increases. The maximum operating rpm of a bearing is inversely proportional to the bearing diameter, which means that a smaller diameter allows a higher operating rpm. Another factor affecting the critical speed is the mass of the impeller. The lighter the impeller, the higher the critical speed of the shaft-impeller construction. Due to the above reasons, impellers intended to be mounted directly on a motor shaft are generally made of light-metal alloy or a composite material.
A typical material choice for heavy-duty composite structures is a so-called prepreg material generally comprising woven carbon fiber preimpregnated with epoxy resin. This material is stored in a sheet inventory as the curing of the resin needs an elevated temperature. During manufacture, the impregnated sheet is first heated to room temperature and laminated in a mold. The mold is placed in a vacuum bag, and curing takes place in an autoclave. This process is employed to manufacture components used in the most demanding aircraft structures, for instance. Components manufactured from prepreg materials are quite expensive due to the great number of work hours and costly raw materials. Furthermore, the strength of a composite structure is dependent on the fiber content in the structure. Typically, components made from prepreg materials can reach a fiber content of 60%, whereby the structure is principally comprised of carbon fiber.
Another manufacturing method used in the art is the RTM process (Resin Transfer Molding). In the RTM process, the fiber cloths used as reinforcement are placed in dry form into the mold. The mold is closed, and the resin is injected under pressure into the mold. This method is quicker than the prepreg process, and readily allows the construction to be made from fiber cheaper than carbon fiber. Conversely, the fiber content is typically only 40%, inasmuch as the product structure is basically comprised of the resin.
As described above, RTM is a manufacturing method using a closed mold. More specifically, the resin is injected into a space defined by mold walls, whereby the method is based on the infusion of liquid resin into the dry reinforcing fabric under imposed pressure. The applications of the RTM method include structural elements having a complicated geometry and typically operating under high stress. As reinforcing materials the RTM method can use almost all dry fabrics and cloths, carbon fiber reinforcements inclusive. The matrix materials can respectively be 1- or 2-component epoxy, vinyl or polyester resins. The process uses closed molds, and the reinforcements are generally preformed prior to the infusion step. The benefits of the method are a high degree of construction integration, high quality of tolerances and surface structure, particularly combined with possibility of utilizing a high degree of automation.
In the prior art, some arrangements for manufacturing composite impellers are known; however, implemented using a prepreg process. One embodiment of a composite impeller, which is directly mounted on the electric motor shaft, is described in patent publication FI 101564 (Hulkkonen et al.). Therein the impeller is made of a composite material substantially comprised of carbon fiber. The manufacturing method is based on a prepreg process. Manufacture of impellers by way of this approach is highly labor intensive, because the preparation of a thick laminate structure requires plural intermediate vacuum pumping cycles. In the manufacture, this means that after a given thickness of lamination has been attained, the structure must be inserted in a vacuum bag with the help of which the layers are compressed firmly against each other. Another worktime-increasing factor arises from the machining of the cured impeller blank to final dimensions. Machining is labor-intensive and cuts fibers. Particularly problematic are severed fibers at the leading edge of the impeller vane that meets the gas flowing at a high velocity, whereby it must be noted that the flow frequently carries along a certain amount of particulates.
Another prepreg manufacturing process is based on embedding an aluminum honeycomb structure or foamed core material in thick constructions. This approach generally used by aircraft manufacturers. For the manufacture of a blower impeller, this method is described in patent publication U.S. Pat. No. 6,402,467 (Godichon et al.). According to this publication, the cored inlet/rear disks and the vanes of the impeller are manufactured separately. After the completion of these components, the necessary machining steps are carried out. The components have guiding surfaces to facilitate the bonding of the components together. Vacuum bonding is employed to finish the impeller. Generally, a compressive tool is required to secure a homogeneous bonding pressure. However, the weakest point in a structure assembled from separate elements is always the bonded seam. To bond seams capable of enduring high stresses in volume series with a consistent quality necessitates the use of expensive equipment and, generally, a post-production qualification step based on some nondestructive test method.
Both prior-art manufacturing methods described above are highly labor-intensive. Moreover, certain other problems have occurred in practical applications. In comparison with metallic impellers, all carbon-fiber composite structures are generally hampered by inferior endurance under erosive conditions. This is caused by the high velocity of gas impounding against the impeller. Generally all practical applications employing blowers or compressors have a certain amount of particulates flowing along with the gas stream. Solid particulates impinging on the composite structure cause abrasive wear that eventually deteriorates the impeller. A remedy to this problem has been sought from the use of metallic reinforcements at the most critical points of the impeller.
In the prior art, manufacturing methods have been extensions to conventional techniques. In patent publication U.S. Pat. No. 6,402,467 is described an embodiment having a steel guard member placed on the leading edge of the impeller vane. Another similar structure is disclosed in patent publication U.S. Pat. No. 6,264,430 (Hulkkonen et al.), wherein the trailing edge of the vane has respectively been reinforced with a metal shield plate. In practice, the above-described embodiments have been problematic. Particularly due to differences in thermal expansion coefficients, high rotational speed and the like reasons, the shields/guards have separated under use thereby causing serious damage.
It must further be noted that prepreg materials also impose constraints to the product geometry, since the prepreg itself as well as the core material are rather sheet-like thus being incompatible with the lamination 3D structures. Resultingly, prior-art embodiments have been hampered by plural disadvantages that have retarded actual advancements in the development of composite material impellers.